1. Field of Invention
The present invention relates to mapping the open natural fractures in the petroleum reservoirs, more particularly identifying their location and their orientation in the existing oil fields.
2. Description of the Prior Art
In most of the carbonate and certain sandstone reservoirs, natural fractures are encountered that are open and control the directional permeability and the effective flow pathways for the hydrocarbons. Mapping these fractures and their orientation is the key to the economic recovery of hydrocarbons from these reservoirs. At present, natural fracture characterization is of increasing importance, since the industry is venturing into increasing their producible reserves from the existing fields that are showing production decline.
Natural fractures in the subsurface rocks are usually vertical and are mostly found in the formations that have gone through structural deformation or have experienced regional stresses. These fractures commonly terminate at lithologic discontinuities within the reservoir formations. These fractures can be closely or widely spaced and irregularly distributed. Quite often, swarms of fractures are encountered with unfractured intervals in between. Economic hydrocarbon production from the fractured reservoirs requires an optimal access of the wellbore to the open fractures. This makes it extremely important that an accurate map of the open fracture system should be available prior to any field infill and development program.
In many cases, fractures are difficult or impossible to map adequately by using currently available technologies. Physical measurements through cores and well logs are limited to the vicinity of the wells drilled in the reservoir. The density of sampling the reservoir rock using cores and well logs quite often is not sufficient to provide any useful information regarding the orientation and the location of the fractures. This is due to two main characteristics of the majority of the wells that are drilled:
1) Both the wells and the fractures are generally vertical and parallel to each other; and
2) The wellbore is smaller than the fracture spacing between the larger fractures.
Horizontal drillingxe2x80x94where the cost of drilling a well is highxe2x80x94has to be designed to take the full advantage of the natural fractures that are open, by mapping their location and their orientation. Since a single horizontal well is limited in producing from a few layers of the reservoir, it is important to identify the part of the reservoir from which the production can be optimized, prior to drilling the well. This requires that the specific fractured beds should be identified prior to any drilling commitments.
This invention uses the measurements of the nonlinearity of the fractured rocks to the seismic waves that are transmitted through them. Two seismic signals are used. One is a high-frequency seismic signal (xe2x80x98carrierxe2x80x99 wave) transmitted from a wellbore, which penetrates the reservoir and travels through the fractured rock and is recorded by the receivers in another well. The second is a seismic impulse that is generated using a surface source. The seismic impulse can be generated using marine water or air-guns for offshore seismic methods, and land seismic impulsive sources like dynamite charges or weight drops for land seismic operations.
The stress generated by the high-powered compressional pulse alters the opening of the fracture or of a conductive subsurface fault. The changes in the width opening of the fracture or a subsurface conductive fault, due to fracture nonlinearity, change the transmission properties of the high frequency xe2x80x98carrierxe2x80x99 wave. Observing the changes in the phase and the amplitude of the xe2x80x98carrierxe2x80x99 wave, one can detect the presence of the fractures and their location. Since the change in the amplitude of the xe2x80x98carrierxe2x80x99 wave signal is coincident with the arrival of the acoustic pulse at the fracture, it provides us the travel time of the seismic acoustic pulse from the surface source to the subsurface fracture location. The surface source can be deployed in any surface pattern of source locations and the travel times measured, thus providing us the location of the subsurface fracture or a conductive fault. The maximum stress across the open fracture is related to the direction of arrival of the seismic acoustic pulse. The stress is maximum when the acoustic pulse arrives at the fracture at near right angles to the fracture orientation, and minimum when the acoustic pulse arrives parallel to the fracture orientation or in the same plane as the fracture itself.
By moving the surface source, which could be a marine vessel that is equipped with air-guns or water-guns, or a dynamite charge deployed for land seismic that can be detonated at any surface location, recordings can be made at different surface locations and measurements of the amplitude and the phase changes of the xe2x80x98carrierxe2x80x99 wave made, to determine the orientation and the location of the fractures.
Since practically all the subsurface fractures are vertical, the fracture width of the open fractures is not affected when the surface source is directly above them. The change in the width of the fracture is maximum when the surface source is at or near right angles to the fractures and at a distant offset, so that the xe2x80x98modulationxe2x80x99 seismic impulse is arriving at the fracture at a wide angle.
Once the fracture orientation is established, their location can be determined by moving the surface source in a straight line at right angles to the fractures. The arrival times from the surface source to the subsurface fracture are determined and calculations made to map the fracture location.
Briefly, the present invention provides a new and an accurate seismic method of mapping the orientation and location of the open natural fractures that are common in the hydrocarbon reservoirs. Two discrete seismic signals are used One is a high frequency signal interference to in the description as a xe2x80x98carrierxe2x80x99 signal which is in the order of ten times higher frequency than the highest frequency in the acoustic pulse which is termed as a xe2x80x98modulationxe2x80x99 signal. The xe2x80x98carrierxe2x80x99 signal is transmitted using a seismic source located in a wellbore with multiple receivers located in an adjacent wellbore. More than one wellbore can be used for receivers to listen simultaneously and each wellbore can have multiple receivers, each receiver with independent output. The borehole source can be moved up and down in the wellbore to cover different formations in the reservoir that may be fractured.
The lower frequency seismic impulse source is located on the surface and can be easily deployed in any geometric pattern that is considered necessary to map the location and orientation of the fractures. Normally the surface sources can transmit higher energy seismic signals compared to the downhole transmitter, their signal strength can be in the order of one hundred times larger. Additionally, the lower frequencies are less attenuated as they travel through the earth. So the level of the xe2x80x98modulationxe2x80x99 signal available at the subsurface fractures can be very much larger than the high frequency xe2x80x98carrierxe2x80x99 generated by the downhole source.
Experiments in rocks show a large nonlinear elastic wave response, far greater than that of gases, liquids, and most other solids. The large response is attributed to structural discontinuities in the rocks such as fractures (P. A. Johnson and K. R. McCall, Los Alamos National Laboratory, Los Alamos, N.Mex.). This nonlinear wave behavior implies that as the seismic wave propagates through the rock there is a local increase in the density and modulus during compression and a local decrease in density and modulus during rarefaction. This generates an elastic nonlinear behavior of the rock and causes interaction between acoustic signals, which propagate through the rock simultaneously. This effect is cumulative in a fractured rock.
A large amplitude, low frequency acoustic impulse squeezes the open fractures during the compression cycle and opens them during the rarefaction cycle. The transmission of the xe2x80x98carrierxe2x80x99 wave through the swarm of fractures is affected by the compression and rarefaction cycles of the xe2x80x98modulationxe2x80x99 wave. Since the frequency of the xe2x80x98carrierxe2x80x99 wave is higher by roughly a factor of 10 or more, and the signal strength of the low frequency xe2x80x98modulationxe2x80x99 wave is considerably stronger. The interaction of the two can be measured. This interaction of the two waves as they are transmitted through multiple open fractures produces amplitude and phase changes in the xe2x80x98carrierxe2x80x99 wave. The effect, as the seismic signal travels through multiple fractures of a swarm is cumulative.
The changes in the amplitude and the phase of the xe2x80x98carrierxe2x80x99 wave due to the nonlinear changes of the transmission characteristics of the fractures when the fractures are squeezed by the acoustic impulse are measured. These changes in the amplitude and phase of the xe2x80x98carrierxe2x80x99 wave are indicative of the angle at which xe2x80x98modulationxe2x80x99 wave is intersecting the fractures. Maximum changes in the amplitude and the phase will occur when xe2x80x98modulation xe2x80x99 wave is at right angles to the fractures and zero or near zero difference when the xe2x80x98modulationxe2x80x99 signal is arriving from directly above or at a direction that is parallel to the fractures.
The measurements and the analyses of the xe2x80x98carrierxe2x80x99 wave signals recorded by the receiver array and transmitted by the downhole source are made for all the downhole source locations. The output from each source location is recorded by the receiver array, which may be 100 independent receiver signals, sampling the receiver well every 5 or 10 feet. The receiver array is designed to be long enough to provide the vertical coverage for the formations of interest. Each data set, when completed, will be a matrix xe2x80x98nxe2x80x99 number of downhole source locations 5 feet or 10 feet apart recorded in all the receivers in the receiver array, that may be 100 independent receiver signals. The xe2x80x98nxe2x80x99 number of downhole source locations will be enough to vertically sample the formations that are part of the reservoir suspected to have open fractures.
This whole recording procedure is repeated for multiple surface source locations, deployed in a geometric pattern. For this description, a simple pattern is used to explain the concept, in real life any other suitable pattern can be designed; the knowledge for such a design is known in the current art.